The present invention relates to a direct sequence code division multiple access system in spread spectrum communications and, more particularly, to a spread spectrum receiver and transmitter that spreads an input signal by both short-term and long-term spreading codes (hereinafter referred to as short and long codes, respectively).
In recent years, a variety of spread spectrum systems have been studied for more effective frequency utilization in digital mobile radio communications (M. K. Simon, J. K. Omura, R. A. Scholtz and B. K. Levitt, "Spread Spectrum Communication", Computer Science Press, 1985). In particular, a DS-CDMA (Direct Sequence-Code Division Multiple Access) system is relatively simple in configuration and studies have been continued with the goal of putting it to practical use. In the application of the DS-CDMA system to, for example, a cellular mobile radio communication system, the same short code can be used in adjacent cells when different long codes are assigned to them.
In FIG. 1 there is illustrated a prior art example of a transmitter in the DS-CDMA system. A digital signal s(m) is fed via an input terminal 11 to a baseband modulator 12, which uses the digital signal s(m) to generate a baseband modulated signal b(n). The baseband modulated signal b(n) is applied to a multiplier 14A forming a spreading part 14, wherein it is spectrum-spread by being multiplied by a short code SC.sub.S that is fed from a short code generator 13.sub.S. The multiplied output is further fed to another multiplier 14B forming the spreading part 14, wherein it is again spectrum-spread by being multiplied by a long code SC.sub.L from a long code generator 13.sub.L. The chip periods of the short and long codes SC.sub.S and SC.sub.L are both T.sub.C, and the short and long code generators 13.sub.S and 13.sub.L operate on a clock signal CK of a clock frequency 1/T.sub.C which is generated by a clock signal generator 17. A baseband modulated signal b.sub.sp (n), which is the output from the multiplier 14B, is applied to a multiplier 19, wherein it is up-converted to the RF frequency band by being multiplied by a carrier signal CW from a carrier signal generator 18, and the multiplier output is amplified by a transmitting amplifier 21, thereafter being sent as a transmitting modulated wave from an antenna 22.
The short code SC.sub.S has a code period of the same length as that of the symbol period T.sub.S of the baseband modulated signal b(n) as shown in FIG. 2 and spectrum-spreads respective symbols b(1), b(2), . . . . On the other hand, the long code SC.sub.L has a very long period T.sub.L corresponding to tens or hundreds of symbol length and is used to randomize signals received from other cells (or zones). The long code is usually a long-term PN (Pseudo Noise) sequence, and the same cell is assigned the same long code and different cells different long codes. Since different long codes have very low correlation, they can be used to randomize received signals from other cells. The short code generator 13.sub.S has, for example, a well-known configuration which EXCLUSIVE ORs outputs from at least two desired shift stages of a shift register and feeds the result of the exclusive ORing back to the input of the shift register. Letting the number of shift stages of the shift register be represented by K, a (2.sup.K -1)-chip pseudo noise code (PN code) which repeats itself with a (2.sup.K -1)T.sub.C period can be generated by driving the shift register with a clock signal of a 1/T.sub.C -chip rate. The long code generator 13.sub.L can be identical in construction with the short code generator 13.sub.S, except that the number of shift stages K is sufficiently larger than that in the latter.
In FIG. 3 there is shown in block form a prior art example of a receiver in the DS-CDMA system. Incidentally, the propagation is assumed to be a two-path Rayleigh fading model and, therefore, its operation will be described on the assumption that the received wave is based on a two-wave model consisting of a direct path and a delayed path. In the first place, the received wave arrives at an antenna 25. The received wave is amplified by a low-noise amplifier 26 and multiplied in a multiplier 28 by a carrier signal CW from a carrier signal generator 27, thereafter being fed to a low-pass filter 29. This operation or manipulation corresponds to down-converting, and the low-pass filter 29 outputs the spread-spectrum baseband modulated signal b.sub.sp (n), which is applied to an input terminal 3.sub.IN of a multipath separating part 30. The spread-spectrum baseband modulated signal b.sub.sp (n) is branched by a hybrid circuit 31 to two paths corresponding to the two propagation paths and input into despreading parts 32.sub.1 and 32.sub.2. A multiplier 32A.sub.1 forming the despreading part 32.sub.1, multiples the spread baseband modulated signal b.sub.sp (n) by a short code SC.sub.S from a short code generator 33.sub.S and provides the multiplied output to another multiplier 32B.sub.1 forming the despreading part 32.sub.1. The multiplier 32B.sub.1 further multiplies the input by a long code SC.sub.L from a long code generator 33.sub.L and provides the multiplied output to an integrator 35.sub.1, which accumulates the latest multiplied results of the same number as the chip number of the short code. In other words, the integrator 35.sub.1 acts just like a low-pass filter that outputs a mean value of a predetermined number of multiplied outputs. These operations corresponds to despreading. These spreading codes SC.sub.S and SC.sub.L have a high auto-correlation and no desired signal can be extracted without coincidence of their timing in transmission and reception. The short code generator 33.sub.S and the long code generator 33.sub.L are driven by a clock signal CK of a clock frequency 1/T.sub.C which is generated by a clock signal generator 39.
Assuming that the spreading codes SC.sub.S and SC.sub.L of the direct path coincide in timing with the spreading codes SC.sub.S and SC.sub.L produced by the short code generator 33.sub.S and the long code generator 33.sub.L, the integrator 35.sub.1 extracts a path component of the direct path, which is provided as a despread baseband modulated signal b.sub.1 (n) to a terminal 3.sub.1. Similarly, a multiplier 32A.sub.2 forming the despreading part 32.sub.2 multiplies the spread baseband modulated signal b.sub.sp (n) by a delayed short code SC.sub.S from a delay circuit 36.sub.S and provides the multiplied output to another multiplier 32B.sub.2 forming the despreading part 32.sub.2. The multiplier 32B.sub.2 further multiplies the input multiplied output by a delayed long code from a delay circuit 36.sub.L and provides the multiplied output to an integrator 35.sub.2, which provides a despread baseband modulated signal b.sub.2 (n) to a terminal 3.sub.2. These operations correspond to despreading. When the spreading timing in the received delayed path of the short and long codes coincides with the timing of the delayed short and long codes SC.sub.S and SC.sub.L, a path component of the delayed path is extracted by the integrator 35.sub.2 and provided as the despread baseband modulated signal b.sub.2 (n) to the terminal 3.sub.2 of the multipath separating part 30.
The hybrid circuit 31, the spreading parts 32.sub.1 and 32.sub.2, the integrators 35.sub.1 and 35.sub.2, the delay circuits 36.sub.S and 36.sub.L, the short code generator 33.sub.S and the long code generator 33.sub.L constitute the multipath separating part 30. A diversity type detecting part 40 inputs thereinto despread baseband modulated signal b.sub.1 (n) and b.sub.2 (n) for the respective propagation paths, provided from the integrators 35.sub.1 and 35.sub.2, then performs diversity detection and outputs the resulting digital signal s(m) to a terminal 41. A possible configuration of the diversity type detecting part 40 is one that combines input signals after differential detection and makes a hard decision.
The above receiver randomizes signals from other users using different short codes in the same cell wherein users share the long code SC.sub.L, that is, randomizes interference signals, besides it randomizes multipath components of a desired signal delayed by different time intervals. These randomized signals are added as noise to the despread baseband modulated signals b.sub.1 (n) and b.sub.2 (n), leading to an increase in the total amount of noise power. If the interference signal components could be canceled from the despread baseband modulated signal by providing the diversity type detecting part 40 with an interference canceling capability, an improved transmission characteristic could be obtained by suppressing the above-mentioned increase in the total amount of noise power. Since the long code has a high auto-correlation, however, multipath components are randomized by the long code when they are delayed even by one chip relative to signals from other users in the same cell assigned the same long code and a signal of a desired signal; hence, these signal components cannot be canceled by the interference canceler.
As another example of the DS-CDMA system that employs the short and long codes, see, for example, U.S. Pat. No. 4,969,159, that uses short and long codes of different chip rates. This is based on the premise that the receiver performs despreading by the short code through the use of a SAW filter. Since the scale of the SAW filter increases with the period length of the short code, it is customary in the art to cut the period length of the short code used to 1/8 the data bit period so as to decrease the scale of the SAW filter and reduce power consumption. At the same time, a long code of a period (15/8 times) longer than the data bit period is used to acquire a large spreading gain. In this system, the period of the long code is 15 times longer than the period of the short code and the chip period of the long code is set at 127 times the chip period of the short code. Since in this system the period of the long code is about twice the data bit period and the chip number of the long code is 15, appreciably smaller than the chip number 127 of the short code, the effect of randomization by the long code is lessened. Therefore, different pairs of long and short codes of low cross correlation cannot be selected in numbers for each cell.